Road drifts not only serve as a road crossing but should also function as a sand dam and riverbed stabilizer. Non-vented drifts 1 can act both as low-volume traffic conduits and water-retaining structures. They help capture and store floodwater and retain it for future use. Unlike vented drifts, non-vented drifts are not equipped with culverts. Drifts provide good opportunities to retain water from ephemeral rivers because they act as sand dams (see Box 8.1).
Box 8.1: Sand dams
Sand dams reinforce what sandy rivers are already doing: storing water in the sand. A retaining wall across the riverbed enables the accumulation of sand upstream, increasing sand storage capacity and water retention.
It is important not to have vents (culverts) in these dry river crossings. Due to the absence of culverts in the drift, coarse sand and gravel will accumulate in the riverbed upstream of the drift. This creates a small manmade aquifer of sand and water. Coarse sand and gravel have large spaces between their pores, which can make for up to 35 percent of total sand volume. This means that up to 35 percent of the volume of sand/gravel can be used to store water. Thus, a non-vented drift builds on the natural capacity of a sandy riverbed. The newly deposited material will store floodwater and make it available during the dry season. The water retained in the riverbed will also connect to and feed aquifers on the banks of the river. The extent of this effect depends on local topography and geology.
Water-retaining sand dams come at no additional cost. In fact, they are even cheaper than the alternative option, i.e., vented drifts equipped with culverts. The exact cost of non-vented drifts depends on their height, which also determines their capacity to retain water.
Table 8.1. Drift construction costs per meter in Makueni County, Kenya 2015
|US$ per meter|
|Drift type 1: Large drift, foundations excavated at maximum depth of 1.5 m and elevated 0.3 m above the existing sandy riverbed.||1240|
|Drift type 2: Large drift, constructed on bedrock, elevated 0.5 m to 1.2 m above the existing riverbed.||760|
|Drift type 3: Small drift, constructed on normal, ordinary river channels. Little or no elevation above the existing riverbed level. Depth 0.5 m to 1.0 m.||475|
|Type 4: Small drift (road slabs), constructed on bedrock or swampy plains. Little or no elevation above the existing riverbed level, maximum depth 0.5 m.||330|
Non-vented drifts also provide other water management benefits. The first is the stabilization of the upstream riverbed. Depending on the lay of the land, non-vented drifts make it possible to divert water from the riverbed—either perennial flows or short-term floods or spates—using gravity upstream of the drift. This would be difficult where the riverbed is not stabilized and smooth but is instead rutted and incised.
Culvertless drifts also cause less damage to the riverbed immediately downstream of the road crossing, since water will not spout through the culverts during flood events to erode the area downstream of the drift. Water now has the chance to cross over the entire width of the drift, reducing damage to land downstream. As river crossings, non-vented drifts are more reliable and predictable and are much cheaper than bridges in their function as low-volume roads. During flood events, however, they are impassable for the duration of the flood, whereas vented drifts are passable (unless they are affected by blocked flotsam and uncontrolled flooding). Downtime on non-vented drifts can be reduced by placing pointed markers alongside the drift to guide vehicles across during low floods.
A possible solution to making culvertless drifts passable during flood events is to add a vented drift on top (Figure 8.1). However, prior analysis is required for roads with different types of traffic volumes to determine whether this construction would be economical and necessary. As mentioned, because floods are mostly limited to a few days a year, culvertless drifts are suited to low-volume routes. For high-volume roads, this combination of structures can be better assessed for suitability on a case-by-case basis.
Figure 8.1: Sandwich drift to ensure passability
Drift = basic structure on the riverbed